The present invention relates to the general field of servovalve actuation systems. The invention notably relates to an actuation system for an aircraft turbojet engine.
A turbojet engine typically comprises servovalve actuation systems, intended for controlling the variable geometries of the turbojet engine.
By  variable geometries  of the turbojet engine, are meant here the units of the engine or of its environment, the dimensions, shapes, positions and/or velocities of which may be modified, depending on detected events or on operating parameters of the engine. Examples of  variable geometries  are stator vanes with variable pitch of compressor rectifiers, discharge valves of the compressor, turbine vane tips and a fuel meter.
Typically, these actuation systems are monitored by the electronic control module of the engine control system or EEC (for  Electronic Engine Control ), so as to adapt the variable geometries to the scenario of the flight. The control is ensured via feedback loops.
Thus, it is well understood that the monitoring of systems for actuating variable geometries in a turbojet engine is crucial in terms of availability and yield of the turbojet engine.
Degradation of these actuation systems are notably expressed by non-compliant positions and/or dimensions of the units of the turbojet engine with the controls of the ECU under established operating conditions, or by slow response of these units to these controls under transient conditions. These degradations are warning signs of failure since generally in a first phase they are either compensated by feedback loops of the actuation systems or without any notably consequence except for reconfiguration (for example a change in the active control channel).
However, after some time, when these degradations persist and worsen, they may no longer be compensated since the control equipment is limited. Thus, these degradations may have the consequence of making the turbojet engine inoperative or unperforming. This may be expressed by a failure message transmitted by the ECU. The detection of these deteriorations is therefore too late, since it is only achieved when the actuation system has failed.
A particular degradation type is the drift of the quiescent current of the servovalve, as shown in FIG. 2.
FIG. 2 is a graph which illustrates an operating curve of a servovalve. More specifically, the graph of FIG. 2 illustrates the time dependent change in the hydraulic flow rate Q delivered by the servovalve 3 to an actuator, depending on the control current Icmd. In FIG. 2, Imax represents the maximum value of the control panel Icmd which the ECU is cable of providing.
The curve 20 corresponds to a rated condition of the servovalve and shows that the quiescent current of the servovalve, i.e. the control current required for maintaining the actuator in a determined position, has a non-zero value I0. In the rated condition, the current Imax however gives the possibility of providing the maximum hydraulic flow rate Qmax.
The curve 21 illustrates a non-rated condition of the servovalve, for example a condition after ageing. It is seen that the quiescent current has value I1 greater than I0. In other words, there has been a drift in the quiescent current of the servovalve. Further, in the non-rated condition of curve 21, the current Imax no longer gives the possibility of providing the maximum hydraulic flow rate Qmax. In other words, the curve 21 corresponds to a degraded condition of the servovalve.
The detection of such a degraded condition is known. However, during the detection, the performances of the turbojet engine have already been substantially affected.
Therefore, there exists a need for having an efficient monitoring method for a system for actuation of variable geometries of a turbojet engine, notably in order to be able to deliver a maintenance notification for this actuation system before the turbojet engine is made inoperative or unperforming.
A similar need exists in other applications of a servovalve actuation system.
Document US2009/0306830 describes a method for monitoring a valve system in which a switching time of a valve element is measured between two positions. If this duration exceeds a predetermined threshold, the method determines whether this exceeding is due to the wear and tear of the system or to general conditions. The general conditions are for example represented by the control current of a solenoid valve. The control current is compared with a reference value. However, this document does not describe that the relevant control current is an average value of the control current during the detection of a stabilized condition, the stabilized condition being detected by verifying that the measured position of an actuator remains constant. On the contrary, the monitoring method described requires the switching of the valve elements between two positions.